The present invention relates to power recovery from arrays of photovoltaic cells.
Photovoltaic (PV) cells have existed for many years and are well described within the art. Summary features of a PV cell are a device that includes a barrier semiconductor junction capable of converting energy from impinging photons into a current dispensed into an external load (see FIG. 1). In photovoltaic power systems it is common practice to wire cells in series strings (see FIG. 2). Series connection minimizes wire power loss by maximizing voltage and minimizing current. Under ideal conditions the photocurrent in each cell closely matches. Under real operating conditions photocurrent may vary greatly between individual cells, or groups of cells. Because all cells are in series, the entire string current is then limited to the cell with the lowest photocurrent.
In order to address this problem, PV strings are commonly divided into substrings, where a bypass diode connects across each substring (see FIG. 3). A substring may consist of a single photovoltaic panel or more typically a cell column or cell column pair within a photovoltaic panel. When photocurrent in one substring falls significantly below string current, the associated bypass diode forward biases, shunting string current around the associated substring. While this preserves the bulk of string current, it results in a total loss of available power from the affected substring as well as loss from the forward voltage drop of the bypass diode.
An alternative to the use of simple passive diodes connects each substring to an independent DC voltage boost converter attached to a common parallel DC bus (see FIG. 4). DC voltage boost converters harvest PV substring power by presenting one impedance to each substring, and a different impedance to the load. A boost converter (see FIG. 5) operates by periodically toggling SPDT switch SW1 between the BUS− and BUS+ connections. SW1 connected to BUS− builds current in L1. SW1 connected to BUS+ discharges L1 current into C1. Typically, SW1 may be constructed using an active switching device such as a MOSFET transistor between L1 and BUS− and a Schottky diode, between L1 and BUS+ (see FIG. 6). In such implementations, D1 also protects against reverse current flow from BUS+ through the PV substring to BUS−.
For L1 sufficiently large to support continuous conduction across each switching cycle, and assuming zero losses, average current delivered to the bus is:IBUS—AVERAGE/IPV—AVERAGE=(TPERIOD−TON)/TPERIOD Control electronics may vary TPERIOD, TON, or both so as to set average PV current, IPV—AVERAGE, at the maximum load power point for the attached PV substring. The control may find the maximum power point by varying ratios of TON to TPERIOD while measuring PV substring terminal voltage and current until a maximum is found.
Disadvantages of boost converters tied to a common parallel bus include: the voltage drop through diode D1; low bus voltage and impedance results in high wiring current and associated power losses; cumulative wiring distribution losses of a parallel network; and catastrophic failure of the network when one element fails short. The percentage power loss through diode D1, and external wiring may be reduced by transforming the output voltage by common means, e.g., a flyback topology, or the addition of a second power conversion stage, each of which adds complexity.
In a parallel wiring scheme, loop current builds with each successive segment approaching the load. For equal length and diameter wires in each segment and equal currents from each source, the power losses in each wiring segment build up quadratically (see FIG. 7). Given ten sources, the wire segment from the furthest to second furthest source loads with 1/10th the current and 1/100th the power loss of the nearest source to the load. In order to avoid excessive power loss and fire hazards, wiring cross-section must either scale from the furthest to closest source, or wire must be over-provisioned for all sources other than the closest. By comparison, current through a series wiring scheme is constant in each segment. Wiring losses per unit length are therefore also constant allowing safe use of a constant cross-section without over-provisioning (see FIG. 8).
Prior solutions include the use of AC inverters in place of simple DC voltage boost converters (see FIG. 9). PV AC-inverter-per-substring schemes mitigate some of the wiring loss issues by operating at a higher voltage/lower current than typical DC voltage boost; most commonly at an AC main value of 105-220 VAC. However, the wiring mechanics, heating, and power loss problems remain the same as for the DC case.
AC single and/or split phase inverters at each substring suffer a further requirement for substantial energy storage. Over the course of a single half cycle load power varies from zero to maximum and back to zero while the PV source power is constant. In order to fully utilize PV source power, a minimum of 32% of the energy delivered in a single half cycle must be accumulated by an energy reservoir. For 60 Hz systems, the minimum energy store required to maintain constant load on the PV source is 2.7 mJ/Watt power input. This substantial energy storage is economically practical only using capacitors, typically of wet electrolytic construction.
Unfortunately, wet electrolytic capacitors exhibit high internal Effective Series Resistance (ESR) which causes internal heating. They also perform poorly in cold environments, exhibiting much higher impedances than at room temperature. They are also characterized by high failure rates and short expected lifetimes aggravated by exposure to moderate or high temperatures. The poor expected lifetime and environmental performance of wet electrolytic capacitors presents a serious problem when used in products such as outdoor PV equipment that is exposed to the elements, has a long expected service life, and a high cost to service due to limited accessibility.
Moreover, in order to protect utility service personnel, AC microinverters tied directly to the premise utility must include a critical safety feature wherein they halt power generation in event of utility failure. This function is known as anti-islanding. Deployment of many AC microinverters in place of a single centralized inverter multiplies the probability that the critical anti-islanding function will fail. A further drawback of both DC voltage boost and parallel AC microinverters is that an inverter is required on each substring, even if only a few substrings are likely to suffer from uneven illumination.